The invention relates to composite cans formed of paperboard with polymer film and/or metal foil materials, and specifically to a machine and method for seaming a disc-shaped metal closure (referred to herein as a “metal end”) onto the top end of a composite can.
Cans are commonly sealed closed with a metal end that is affixed to the can by a seaming operation. When packaging products that are adversely affected by exposure to air, it is frequently desired to evacuate the inside of the can to remove air, and then to introduce an inert gas such as nitrogen into the can while concurrently seaming the metal end onto the can. The seaming operation entails rolling a curled edge of the metal end and a curled flange of the can together to form a “double seam”. The seaming machine employs a seaming chuck and a pair of seaming rollers to effect this rolling and seaming operation. The can with the metal end thereon is held against the chuck and the seaming rollers roll the curled edge of the metal end and the flange to form the double seam.
A rotary turret type of seaming machine typically is used for seaming metal ends onto metal cans. The machine has a rotary turntable that supports a plurality of chambers spaced about its circumference. Each chamber essentially comprises a cylindrical tube into which a metal can with a metal end crimped thereon is loaded. The chamber's bottom comprises a lifting plate. A seaming chuck is mounted above each of the chambers. The lifting plates are vertically movable relative to the seaming chucks. A cam is mounted beneath the turntable and engages lifters attached to the lifting plates. As the turntable is rotated about its axis, the lifter for a given chamber is moved vertically according to the cam profile to cause the lifting plate to rise and fall, thereby lifting and lowering the can, in order to perform the various operations involved in the seaming process.
Specifically, the turntable has four 90-degree sectors denoted as A, B, C, and D. In each sector, a particular operation is carried out. A metal end is crimped onto the top of the metal can prior to loading the can into the chamber. During sector A the can is loaded onto the lifting plate and the chamber closes. During sector B a vacuum is drawn inside the chamber. The metal end includes stand-off dimples or the like to provide a gap between the metal end and the can to allow gas transfer out of the can. An inert gas is introduced into the chamber as the turntable continues to rotate through sector C. The inert gas flows into the can through the gap provided by the stand-off dimples. During the last sector D the can is raised and the final seaming is carried out, followed by discharge of the can onto a conveyor.
When this type of machine is used to attempt to seam metal ends onto composite cans, a difficulty is encountered. A metal can has sufficient strength to resist the pressure differential that is created between the inside and the outside of the can when the inert gas is introduced at relatively high pressure into the previously evacuated chamber. In contrast, with a composite can, such a pressure differential can cause the can to implode.
The invention is aimed at solving this implosion problem.